The advent of computers in recent years gave rise to the development of what is known as access flooring. Such flooring comprised a modular embodiment of rigid structural floor panels supported on pedestal columns and frequently requiring beam-like stringer members spanned between said columns for edge reinforcement. Typical assemblies were installed on top of a supporting subfloor, thus providing an adequate and secluded space to accommodate an array of power cables and the like beneath the readily accessible floor panels. This underfloor space or cavity also served well as a distribution plenum for conditioned air. Change in location and frequent servicing of computerized equipment in an access floor environment requires repetitive physical handling of the interconnecting cables and is accomplished quite easily by the temporary removal of such modular panels. Subsequently, the underfloor cavity is exposed for complete freedom to perform any task within the maze of previously hidden wiring. When such work is finished, the modular panels fit quickly and easily back into their original position, thereby returning the area to a totally unobstructed and uniform top floor surface.
In view of the weight of computers and other equipment, it was essential that the modular floor panels be substantially rigid, such that when loaded they do not appreciably allow flexure or retain permanent set once flexed, so that the access floor is uniformly flat and all panels are in a common plane. To accomplish this, some of the earlier panels were made of substantially reinforced metal encased wood and heavy metalic castings but this has been appreciably abandoned in favor of lighter weight, high strength metal sandwich panels of which the panel comprising the subject matter of U.S. Pat. No. 3,236,018, to Graham et al, dated Feb. 22, 1966, is an example which is popular and is still being produced and used extensively in the industry.
In recent years, the access flooring industry has expanded in two ways. The first is in the field of high performance, heavy duty panels for specific heavy load areas above and beyond standard computer room criteria. Secondly, the access floor industry has been expanding more and more into office renovation and general office construction for new buildings and other typical lighter load applications. A dramatic shift in the performance requirements for this type of general construction floor has substantially taken place.
Current art has attempted to economically satisfy this need by means of altering panel designs by varying material thicknesses, but sometimes by discarding the standard stringer support provided at the panel edge. Although this readily achieves a lighter weight and more economical product, it inherently introduces an objectionable deficiency, namely, edge-to-edge movement, in the character of the system as viewed by architects and users who are foremost aesthetically minded. Another prominent objection is related to the feel of springiness within a light-weight floor under normal foot traffic. Insecure feelings also arise due to the visible edge-to-edge movement between panels under light equipment use. Although the structural integrity, specifically the ultimate strength, more than satisfies the most stringent general office construction criteria, this objectionable panel-to-panel motion is a restricting factor to the growth and acceptance of the product in the general construction and renovation market.
The industry, to satisfy this need, has developed systems which reduce perimeter movement by adding secondary structures, such as perimeter stringers or complex panel-to-panel hard connecting devices. Although such structures tend to reduce edge-to-edge movement, they directly affect the accessibility and ease of handling the floor system as originally conceived.
Additionally, in renovations and, more importantly, in general office construction, it is desirable to hold the finished floor height of the access floor to a minimum while providing an adequate space or cavity for underfloor cables and to perform as an air distribution plenum. Thicker panels diminish vital underfloor clearance or floor-to-ceiling height. The thickness of the access floor panel is often as much as one-third of the total of this low finished floor height; therefore, an economical panel with needed structural properties, yet thinner in depth, is an advancement in the art and a savings in total building height and cost for new construction, and also provides the ability to maintain adequate minimum floor-to-ceiling height in renovations. To do this, the structural efficiency of the panel has to be dramatically increased over existing art, especially at the perimeter.
Since the development of the aforementioned patented panel, efforts have been made to simplify and also reduce the weight and the amount of metal required in said panels, without reducing, but instead, striving to increase the resistance of the panels to flexure and/or permanent set, especially at the panel perimeter when subjected to static or moving loads. This has resulted in explorations and development, especially in the design, of the structural member which is primarily the lower stress member in a metal sandwich-type floor panel in which it is able to perform integrally with the upper planar member upon which the load is usually imposed.
In such exploration, we have determined that a key factor to resistance to flexure is the reduction of clear straight lines of vision through said sandwich panel and, more importantly, the repeated blockage in all directions within the sandwich. It has been found that several patterns of projections with common strategic dimensional relationships can provide both necessary blockage of clear lines of vision and suitable support of the top sheet to resist localized indentation of the access floor panel. Projections were selected because they combined the benefits of a continuous bottom member with the support obtained from arch-shaped projections to prevent collapse thereof, together with an optimum depth which, combined with the structural and economic efficiency of the section, developed blockage of "see through", thereby providing sufficient section properties to resist deflection by the loads applied.
The strategic dimensional relationship is a combination of considerations for five major characteristics of the projections and their interrelation; namely, (1) depth of projections for needed section modulus and moment of inertia; (2) diameter of projections to obtain their needed depth; (3) distance between the centerlines of projections for sufficient top sheet support; (4) strategic positioning of projections to repeatedly block clear lines of vision throughout the member; and (5) remaining bottom surface material adequate to perform as a stress member and also develop necessary section modulus and moment of inertia. Prior art has failed to combine and/or incorporate one or more of these five structurally significant characteristics and has, therefore, accomplished a less than optimum one-piece structural member which, when combined with a top sheet, does not provide an economical metal sandwich construction of desired comparable strength-to-weight ratio or structural efficiency.
It has been found that specific patterns of several different embodiments of projections, in which at least the major portions of the configuration of said projections are circular and details of which are described fully hereinafter, results in increased rigidity and resistance to flexure to a marked extent. Said projections are formed in sheets of planar industrial material of lighter gauge than now employed in the floor panel of the type shown in said aforementioned U.S. Pat. No. 3,236,018 for improved performance under the same load conditions. The efficiency in performance of the developed core structure has thus been dramatically improved. Such characteristics and features are not found in the prior art, notwithstanding the disclosure of formed sheets having various types of projecting ribs and/or figures of a regular contour pressed from planar sheets and other means which block straight lines of sight across the formed sheet and through such ribs and figures, either because the configurations do not permit sufficient depth of section for strength purposes, are not sufficiently conducive to resist flexure under the required loads existing in access floor use, or are not economical to competively market the same.
The prime object of this present invention is to form, within a single sheet, a structurally efficient combination core and bottom stress member which when affixed to a planar top member surpasses the combined strength, rigidity, and economics of prior art.